Influence of steel slag fineness on the hydration of cement-steel slag composite pastes

Steel slag has the potential to be used as a mineral admixture , but its wide utilization is restricted by the low activity. Grinding may increase the slag's hydration activity. However, little is known about the role of superfine steel slag in the composite cement paste . In this work, the hydration, microstructure, and macroscopic performances of the samples containing ground steel slags were investigated. The results show that most pastes containing superfine steel slag have longer setting time. In addition, the compressive strengths of most mortars decrease, rather than increase, when they contain superfine steel slags. The further investigation reveals that with the fineness increasing, the steel slag paste generates more hydration heat and higher content of Ca(OH) 2 , while the composite paste shows reverse trends. It indicates that although the finer steel slags themselves present higher reactivity, they inhibit the hydration of cement in composite pastes. This inhibition plays a dominant role in most samples, while the filling effect and the increased slag activity seem insignificant in the composite pastes. • The superfine steel slags have higher hydration activity but inhibits the hydration of cement. • The superfine steel slags show better physical filling effect. • The positive effect, together with the negative effect, determines the macroscopic performances.